1. Technical Field of the Invention
The present invention relates an on-vehicle alternator (a AC-generator to be mounted on a vehicle), installed on a vehicle such as a passenger car or a truck, which is driven by an engine via a belt drive system.
2. Description of the Related Art
In recent years, auxiliary-unit component parts such as an on-vehicle alternator, an air conditioning unit and an idler pulley are installed on a motor vehicle and incorporate rolling bearings, respectively. These rolling bearings are probable to suffer severe conditions in vibrations and temperatures. Under such conditions, the rolling bearing has undergone a tissue change in a new pattern accompanied by the occurrence of flaking. This flaking takes place in any area of an inner race, an outer race and balls (or rollers) of the bearing. This flaking has a feature that differs from a commonly known rolling fatigue life of the related art and once flaking occurs, flaking takes place on the outer race surface in an area associated with the ball within a short-time period (in the ratio of approximately 1/100 to 1/1000 times that of the related art).
Further, such a flaking phenomenon is not caused to necessarily occur in a rolling bearing installed on a vehicle and subjected to increased stress such as increased vibration or high temperatures and it was hard to specify what was a cause of such a failure. Therefore, stopgap measures have been taken for the rolling bearing with no clear scientific basis and no full-fledged measure has heretofore been taken to address the issue arising in current conditions. For example, it has been considered in the related art that for a cause of such a flaking phenomenon, water molecules in grease or water resulting from dew condensation are decomposed accompanied by generation of hydrogen ions that penetrate into metallic component parts such as an outer race or balls to cause hydrogen brittleness under which early flaking was caused to take place (as disclosed in, for instance, Japanese Patent Laid-Open Publication No. 2002-227854). However, even with the use of grease adapted to suppress the hydrogen generation, no early flaking has heretofore been addressed as expected.
In particular, among the auxiliary-unit component parts, a vehicular alternating current generator (also called an alternator) has a maximum speed-up ratio and equivalent inertia with a numeric value times the square of a speed-up ratio further increases. Therefore, the bearing of such an alternator had encountered flaking failures in the highest frequency during reliability tests conducted in the related art. Moreover, the auxiliary-unit component parts, installed on a Poly-V belt, whose tension was set to an increased level, or a belt drive system with an auto-tensioner adapted to prevent the loosening of a belt had a tendency of encountering flaking failures in an increased frequency.
By using a rotating fluctuation test applying a rolling bearing with simply repetitive rotating fluctuation as shown in FIG. 6, the present inventors have revealed that the flaking phenomenon can be caused to reoccur on the rolling element. This test has been conducted in a way to rotate the rolling bearing with rotation ripples in the order two times a rotational speed (at a value equivalent to an order of explosions of the engine) by applying the rotation ripples at an average rotating fluctuation rate of 2% (corresponding to a quite common fluctuation rate in such a rotational speed of an actual engine wherein the fluctuation rate varies in a range greater than 30%) upon controlling an electric motor with a view to simulating vibration of the actual four-cylinder engine. FIG. 7 is a view showing conditions such as a rotational speed pattern used in the test. The belt tension was set to reach an average value of a commonly used vehicle.
As a result, a failure could be caused to reoccur on the ball with flaking. Further, a noteworthy point is the finding of the failure, appearing immediately after the occurrence of flaking, with a feature of an indentation formed on a raceway track of the ball as shown in FIG. 8. No increased load was applied to the rolling bearing under the recurrence test condition and no probability occurs in the occurrence of flaking in nature. The indentation (in plastic deformation) is probable to occur in a case where stress resulting from load exceeds yield stress and the indentation tends to occur with further lessened stress when a frictional force is created on an abutment surface (during a defective lubricating condition). As a consequence, the present inventors have made assumption that the lubricating defect has occurred accompanied by the formation of the indentation not because of increased load but because of some conditions. During operation of the bearing, rotation of the ball causes grease to be dragged to form an oil film. However, if assumption is made that due to some condition, a collision takes place between the outer race and the ball under a condition with no presence of a relative speed, no oil film is formed due to the dragging of the ball and, hence, no lubrication takes place between associated component parts with base oil which was supposed to perform rotating lubrication.
The present inventors have made the assumption that a condition, which is hard for an EHL theory (elastic hydraulic lubrication theory) to be established, is caused to occur, like a collision resulting from an appropriate speed component, based on the vehicle and rotation stress causing defective lubrication between a bearing drive wheel (inner race) and the ball accompanied by the formation of the indentation. A further consideration has been taken that damage occurs in a favorable bearing function on the basis of such an indentation accompanied by the occurrence of flaking.
With the rolling bearing used in the on-vehicle alternator, it has been a general practice to allow a clearance between the inner and outer race and the ball to be provided in the order of several micro millimeters so as to avoid an extreme degree of interference pressure from being exerted to these component elements. This minimizes temperature rise and prevents the associated component parts from suffering from seizing due to a contact between the associated metallic component parts resulting from heat deterioration of grease. However, the presence of clearance naturally causes a collision to take place between the inner and outer race and the ball due to vibration. A design of the clearance of the bearing in present days is usually made on the basis of a premise that a on-vehicle alternator is used under high temperature conditions and the design is made so as to avoid the occurrence of seizing even if distortion occurs in the inner and outer races of the bearing. Accordingly, if a bearing is manufactured with a lessened clearance, a new issue arises with the occurrence of a seizing failure.